Flexible pipes for offshore transportation are well known in the art. Such pipes comprise an inner liner which forms a barrier against the outflow of the fluid which is conveyed through the pipe, and one or more armouring layers on the outer side of the inner liner (outer armouring layer(s)). The flexible pipe may comprise additional layers such as one or more inner armour layers to prevent the collapse of the inner liner. Such inner armouring layer or layers are normally referred to as a carcass. An outer sheath may be provided with the object of forming a barrier against the ingress of fluids from the pipe surroundings to the armour layers.
Typical unbonded flexible pipes are e.g. disclosed in WO0161232A1, U.S. Pat. No. 6,123,114 and U.S. Pat. No. 6,085,799.
The term “unbonded” means in this text that at least two of the layers including the armouring layers and polymer layers are not bonded to each other. In practice the pipe will comprise at least two armouring layers, which are not bonded to each other directly or indirectly via other layers along the pipe. Thereby the pipe becomes bendable and sufficiently flexible to roll up for transportation.
The above-mentioned type of flexible pipes is used, among other things, for off shore as well as some on-shore applications for the transport of fluids and gases. Flexible pipes can e.g. be used for the transportation of fluids where very high or varying water pressures exist along the longitudinal axis of the pipe, such as riser pipes which extend from the seabed up to an installation on or near the surface of the sea, pipes for transportation of liquid and gases between installations, pipes which are located at great depths on the seabed, or between installations near the surface of the sea.
In traditional flexible pipes, the one or more outer armouring layers are most often in the form of helically wound steel strips in the form of wires e.g. shaped as profiles, where the individual layers may be wound with different winding angles relative to the pipe axis. The carcass is typically made from wound stainless steel strips.
A pipe of the above type needs to fulfil a number of requirements. First of all the pipe should have high mechanical strength to withstand the enormous forces it will be subjected to during transportation, laying down and in operation. The internal pressure (from inside of the pipe and outwards) and the external pressure (from outside of the pipe) are very high and may vary considerably. Simultaneously the flexible pipe should be very resistant to corrosive fluids. Furthermore, it is important that the weight is kept sufficiently low as a too high weight may rupture the pipe during laying out and further the more weight, the more expensive the transportation.
In general such pipes are expected to have a lifetime of about 20 years in operation.
For obtaining a high strain, high corrosion resistant flexible pipe at least one of the armouring layers, such as the carcass is made from duplex stainless steel.
Duplex stainless steel is a steel consisting essentially of a mixed microstructure of austenite and ferrite most often in an about 50:50 volume % mix, and preferably mix may be about 40:60 to about 60:40 volume % mix of austenite and ferrite. During the production and use of such Duplex stainless steel formation of martensite from the austenite should preferably be avoided, because this may lead to an undesired increase in brittleness and/or a reduced corrosion resistance.
The flexible pipes are normally produced in relatively long lengths such as about 50 m or more, such as about 500 m or more or even in lengths of up to 2 or 3 kilometres or even more. Due to the long length it is necessary to weld sections of duplex stainless steel strips together. Such weldings are usually performed by flash welding. However it has been found that this welding does not provide a sufficiently string and reliable structure.